Allow stack operations to be fused

[tomwhite]

The way stack is currently implemented is slightly different to the other functions in that the number of arguments passed to the blockwise function _read_stack_chunk is just one (the chunk being stored), which is different to the number of arrays that the stack operation in the DAG has as predecessor nodes (the arrays being stacked).

This means that the fusion logic in blockwise breaks in this case. A simple workaround is to mark the stack operation as not fusable, but it would be better to fix this by removing the limitation directly.